First trim the decal to the size you want - a 2mm border around the design is ideal.
Then tease of the paper backing - the tip of a Stanley knife or scalpel is ideal for this.
The carefully put the decal where you want it, taking care not to get any air bubbles trapped, and rub it down starting from one end (the blunt end of a pencil or ball point is suitable for this).
Finally give it a coat of clear lacquer to protect it - printable CD protection spray is ideal.